Applying Adjustment Factor to Free Air Cables used for a Tap.

[Sparky2791]

Working on a project for a client of ours and came to a question regarding applying adjustment factor to cables.

The way the original switchboard was installed in the electric room allowed no room for expansion since it is installed tight up against a block wall. It is a 277/480V, 4000A switchboard and peak demand load history for the building shows they only use 1/3 of its capacity. They are out of breaker space in the switchboard and need room to add additional battery charger loads. We proposed to tap the bus of the existing gear cut a hole in the block wall and feed through to a new 4000A switchboard. We had the manufacturer look at it to verify a bus tap is possible and this was confirmed. I have attached an illustration and photo for reference.

Code reference implied below are from the 2008 NEC ? which is what Pennsylvania still uses.

The question is regarding the tap conductors. These will be installed free air inside the switchboard to through the wall, into the pull box for final connection to the new switchboard. Looking @ table 310.17 500?s are good for 620A (THHN,THWN) free air.
4000A /620A= 6.45 so a total of 7 sets.

However I am wondering if I will need to de-rate the cables per 310.15 (B) (2). If I must de-rate than it is a situation where the total number of cables is determined by the final de-rating factor.

Example ? (let?s assume Neutral is not a current carrying conductor, that?s another discussion entirely)

7 sets of 500?s = 21 current carrying conductors in all. So Table 310.15 (B)(2)(a) tells me adjustment factor of 45%
So using 90degree for de-rating of the 500?s (per 110.14 C)
700A x 45% = 315A
Therefore 4000A/315A = 12.6 Now I need 13 sets of paralleled 500?s??. but wait??..

13 sets = 39 current carrying conductors so now???

700A x 40% = 280A

Therefore 4000A/280A = 14.2. Now I need 15 sets of paralleled 500?s??..so now??..

15 sets = 45 current carrying conductors so now???

700A x 35% = 245A

Therefore 4000A/245A = 16.3. Now I need 17 sets of paralleled 500?s??..and I?ve finally reached rock bottom.

I cannot put the paralleled conductors in raceway within the gear so they will be installed for lengths longer than 24? without separation (exception #3) inside the gear. Am I over thinking this? Not the first time I?ve been ?accused? of doing that if I am .

 

[Smart $]

Code has no ampacity ratings for conductors inside an enclosure. It assumes circuit wiring will always leave the enclosure using a recognized wiring method... and the wiring method is what determines circuit ampacity. As a side note, the inside of an enclosure is not 'free air'.

There seems to be some pertinent missing detail. How are the conductors transitioned from the existing switchgear to the pull box... and how will they get into the new switchgear from the pull box?

 

[Smart $]

FWIW, if you run wireway between existing and new gear, no need for pull box, and no derating for number of conductors where not in excess of 30.

 

[Sparky2791]

Code has no ampacity ratings for conductors inside an enclosure. It assumes circuit wiring will always leave the enclosure using a recognized wiring method... and the wiring method is what determines circuit ampacity. As a side note, the inside of an enclosure is not 'free air'.

There seems to be some pertinent missing detail. How are the conductors transitioned from the existing switchgear to the pull box... and how will they get into the new switchgear from the pull box?

May not be Free air but not in raceway either.

Conductors will land on lugs added to the taps in the bus for the existing gear than through the side of the gear through the hole in the wall into the pull box. From the pull box they will simply connect to the incoming lugs of the switchgear.

 

[Smart $]

May not be Free air but not in raceway either.

Conductors will land on lugs added to the taps in the bus for the existing gear than through the side of the gear through the hole in the wall into the pull box. From the pull box they will simply connect to the incoming lugs of the switchgear.

Unless the pull box attaches directly to the gear enclosures, there has to be a Chapter 3 wiring method used in between.

 

[ron]

Seems like an unfortunate reference in 110.14(C)(1) but it specifically requires you to follow Table 310.16 for ampacities. You will need to use 310.16 for ampacities in the equipment enclosure and you can tap to free air rated conductors outside the enclosure.

 

[tortuga]

It is a 277/480V, 4000A switchboard and peak demand load history for the building shows they only use 1/3 of its capacity. They are out of breaker space in the switchboard and need room to add additional battery charger loads. We proposed to tap the bus of the existing gear cut a hole in the block wall and feed through to a new 4000A switchboard.

A few questions come to mind:
How large are the service entrance conductors?
I think your 'tap' need not be larger than the service entrance conductors.
Would it be not be permissible to size the tap conductors based on the disconnects for the additional load?
Have you ruled out buss duct for some reason?
Cheers